1. Field of the Invention
This invention relates to a feed-screw support structure adapted to absorb the thermal expansion of the feed screw of a saddle, table or the like in a machine tool, in particular, a machining center.
2. Description of the Related Art
In a machine tool which is required to have particularly high dynamic performance, such as a machining center, the feed rigidity must be heightened by supporting both ends of the male screw in such a manner as to preclude any movement in the axial direction, as shown in FIG. 12. Even if a ball screw is employed for the feed screw, the screw will expand with heat if the machine is operated long since the heat generation due to the rolling friction of the ball will cause gradual temperature rise. In view of this, it is the normal practice, in a machine of the type in which both screw ends are fixed, to provide the screw with pre-tension for absorbing elongation. However, in a case where the machine is operated long at high speed, this pre-tension alone cannot absorb the elongation completely, which leads to the generation of an excessive compressive force in the screw, resulting in not only deterioration in accuracy, but also damage to the thrust bearing or an excessively shortened lifetime thereof. To cope with this, various methods of restraining the heat generation in the screw are being practiced. According to one such method, a hollow screw is employed through the central hole of which coolant is allowed to flow. In another method, the heat generation is restrained by means of an oil-mist lubrication system or the like in the ball nut portion.